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I am not adding anything lehalacha, but the siach yitzchok could agree that the old-time garments were worn on the body. However, now that we wear special garments m'yuchad for tzitzis, it would be a bizayon to the mitzva to wear that on the body. (No source, just my own idea.)

This answers a simple question that if the original takanah of Tzitzis was on clothes (it is only today that we wear a special garment to obligate ourselves), you would not be allowed to put it on regular clothes either as one's clothes absorbs sweat.

For Kabalistic reasons (See the Pri Etz Chaim, Shaar Tzitzit, quoted here), the Ari Z"l would wear his Tallit Katan under all his garments. This seems to indicate that he wore them directly on his skin.

The Lubavitcher Rebbe says that R' Chaim Vital was describing a private practice of the Arizal, but (assuming it is literal) it would appear that the Arizal did not have an issue with wearing the Tallit Katan directly on his skin.

The Shulchan Aruch (Orach Chaim 8:11) says that "The main Mitzvah of a Tallit Katan is to wear it on one's clothes, so that he constantly see it and remember the Mitzvot. The Magen Avraham (quoting the Pri Etz Chaim brought above) says that while there are Kabalistic reasons to where the Tallit Katan under the garments, the Tzitzit strings themselves should still be visible (See M"B S"K 25).

Halacha Berurah (8:33) rules that it's good wear something under the tallit kattan in order to separate it from the body, but me'ikar hadin it's permissable to wear the tallit kattan without another garment underneath.